1. Field of the Invention
The invention relates in general to a stator for permanent magnet motors, and particularly to a stator design for a brushless permanent magnet motor that includes a combination slot wedge and tooth locator.
2. Description of the Related Art
Permanent magnet motors include a stator core, which is typically made of a stack of thin, metal laminations. The laminations are usually round, with a central opening. The stator core thus is generally cylindrical in shape, with a cavity extending longitudinally about the central axis of the core. In brushless permanent magnet motors, each stator lamination includes radially-extending openings from the central opening, generally called slots or notches, that are aligned when stacked to receive stator windings, or conductors, wound around the “teeth” formed by the radially-extending openings. The stator core surrounds a rotor, typically consisting of a circular steel shaft or a stack of annular laminations, and a number of permanent magnets are fixed around the circumference of the rotor.
In permanent magnet motors, cogging torque is caused by the combination of two factors, the permanent magnet magnetomotive force and the variation of the air gap permeance between the stator and the rotor. In the design of permanent magnet machines, cogging torque can be an important design consideration. Cogging torque adds unwanted harmonic components to the torque-angle curve, resulting in torque pulsation upon operation of the machine. Although net cogging torque is zero, levels of cogging torque at any given point in time cause noise, power losses and inaccuracies, particularly in servo-positioning drives. Thus, minimizing the momentary cogging torque is desirable. One approach to reducing the momentary cogging torque is to reduce the variation in air gap permeance by, for example, reducing the width of the stator slot openings. Small openings, among other problems, make the insertion of stator windings difficult.
Small openings are particularly a problem when considering power density goals in permanent magnet motors. Power density is determined by the rated power of the motor per unit volume. In most applications, the permanent magnet motor is required to have a high power density. This leads to a high volume of wire in a slot. To address this problem in part, slot wedges have been proposed that hold the stator windings in the slot against the radial force pushing them out of the slot, but such wedges fail to address tangential forces oftentimes experienced by the stator teeth.